Dielectric relaxation of supercritical water and methanol

Abstract

We have developed a microwave spectroscopy that can be applied to various fluids under high temperature and high pressure in the frequency range up to 40 GHz. By utilizing this new technique, we have measured the static dielectric constant and the dielectric relaxation time for water, heavy water, methanol in wide density ranges including their supercritical conditions. To interpret the observed temperature and density dependence of the relaxation time τ D for H 2O and D 2O, we have divided the fluid phase into three regimes: (1) gaseous state, (2) high temperature liquid and (3) low temperature liquid. In the first regime, where the hydrogen bond is of little importance, τ D is simply expressed by the binary collision time, implying that the molecular orientation cannot be changed by the applied electric field as far as the molecule is rotating. In the second regime, the contribution of bound molecules that are incorporated in the hydrogen-bond (HB) network should be taken into account. The relaxation time of the bound molecule is longer than that of the free one by the time τ B that is required to escape from the HB network, and τ B can be described by the librational frequency and the HB enthalpy. In the third regime the escape time should be enhanced from τ B because some molecules once separated from the HB network could be recaptured by it. These three relaxation regimes have been also confirmed for methanol. It is found that, when the inter-molecular stretching frequency is used in place of the librational one in the second regime, the degree of hydrogen bonding estimated from the NMR chemical shift can be correctly predicted from the observed τ D.